10 Tips for better construction photography

04 - Expose your shots properly

Automatic exposure compensation has been one of the most important advances in photography in the last 30 years.  When thru-the-lens built-in light meters were first introduced they freed the photographer from the need for a separate light meter to properly expose each photograph.  The first systems simply showed a needle in the viewfinder that moved up and down as the aperture and shutter speed were varied.  The photographer manually adjusted the settings until the meter needle was centered to get the correct exposure.  These simple needle-matching systems gave way to more and more complex automatic metering systems that analyzed the exposure in multiple zones and compared the exposure in each zone to a standard set by analyzing thousands of professional photographs.  

Although today’s automatic exposure systems on digital cameras are extremely capable, they suffer from a limitation left over from the days of shooting film.  The sensors in modern digital cameras cannot accurately record images that have a wide dynamic range, i.e. a wide range of light values.  If a scene contains objects that are in bright sunlight and objects that are in dark shadows you have a choice of exposing for the highlights or exposing for the shadows.  The camera’s digital sensor will record one or the other, but not both.  This is illustrated in the figure nearby.  In the leftmost image the exposure was set to capture the highlights of the outdoor patio.  The result is that all detail in the couch is lost in the shadows.  Similarly, the middle exposure brings out the detail of the fabric in the couch and the pattern on the pillow, but the outdoor scene is completely overexposed.  The issue is that the camera’s sensor does not have enough dynamic range to expose all parts of the image properly.  

 Film photographers have dealt with this issue for years.  Ansel Adams developed the Zone system to deal with this problem when shooting black and white film.  By carefully controlling the exposure of the film and the film development time, black and white photographers are able to record images that have light values in all ten zones of the zone system.  This means that the scene has areas in which the light value differs by 10 f-stops.  A spot meter pointed at the brightest portion of the scene might read 1/100 sec at f/64 while the same spot meter pointed at the darkest portion of the scene would read 1/100 sec at f/2.8.  Although black and white film can record 10 f-stops of dynamic range, color slide film can only record about 7 f-stops.  Sensors on most digital cameras have a dynamic range similar to slide film.  This means that if you are photographing a scene with widely varying light values, such as the inside of a wall around a window looking out on a brightly lit outdoor scene, you will have to choose which parts of the scene you want properly exposed.  You will need to overexpose the objects outside the window in order that the walls inside the house will be properly exposed.

One trick the pros use to deal with high dynamic range scenes is to capture two or more images of the scene and compose them together in Photoshop.  The rightmost image in the nearby illustration was generated by taking the outside highlights from the image on the left and the shadow detail from the center image.  The resulting image shows good detail over a wide dynamic range of light values that could not be captured in a single photograph.  It helps to use a tripod to steady the camera between shots when you are doing this type of high dynamic range (HDR) photograph.

 In summary, automatic exposure compensation is a wonderful thing, but it can easily be fooled by scenes that have a wide variation in light intensity.  In such cases you may need to switch to manual exposure and set the overall exposure for the photograph using the portion of the scene that is of most interest to you.

 

Ken VanBree is the owner of Imaging Perspective, a Bay Area firm that specializes in as-built construction photography.  Questions or comments are welcome: ken@imagingperspective.com